The present invention relates to rotary drum mixers and, in particular, it relates to the construction and sealing of a scoop chamber or turbine section of a drum mixer relative to a stationary hood.
A variety of rotary drum mixers for mixing product matter in particulate form are known in the art. Rotary drum mixers typically include a horizontally mounted, open-ended drum which is rotated about the generally horizontal central axis of the drum. A stationary hopper may be positioned on or adjacent the mixer for introducing product matter into the drum. The interior of the drum may include structure to mix the product matter to a desired blend as the drum rotates.
In some drum mixers, a turbine or scoop section is fixedly attached to the open end of the drum. The scoop section has turbine blades or scoop blades which elevate product matter during rotation of the scoop section and then provide a gravitationally directed curtain flow of particulate product out of the scoop blades. The curtain flow provides several advantages. First, the curtain flow helps to randomly mix the particulate product in the curtain, and thus facilitates the overall mixing of the drum mixer. Second, the curtain flow provides a excellent location for evenly spraying or coating a substance on the particulate product during mixing. Additionally, the elevating of the product matter provides a raised location to gravitationally remove or discharge product matter out of the drum mixer.
A stationary hood may be used to enclose the drum open end and the scoop section. A discharge chute may be mounted on the hood central to the scoop blades. A product inlet hopper may also be provided on the stationary hood.
One of the primary problems with horizontally mounted drum mixers is the potential for leakage between the rotating drum/scoop section combination and the stationary hood. A seal member may be positioned between the stationary hood and the outside of the rotating drum. The seal member of the prior art rotary drum mixer generally comprises a rubber stationary seal component on the annular flange and a TEFLON-like rotating seal component affixed to the drum contacting the stationary material as the drum rotated. Such rotary drum mixers as described are illustrated, for example, in U.S. Pat. Nos. 3,552,721 and 3,897,934.
For relatively large-sized product matter, the fixed seal member referenced above relies on the flexibility of its rubber component to maintain a closure for radial run-outs as much as 1/4 inch to 3/8 inch around the surface of the rotating member. However, if fine-sized and sub-micron particle sized product matter is being mixed within the rotating drum mixer, the product matter will pass under the fixed seal member. The product matter could then build-up under the stationary rubber-seal and/or pass between the rotating and stationary seal components, thereby compromising the purity of the product matter being mixed and the work environment outside of the rotary drum mixer.
Additionally, should a build-up of product occur on the rotating seal member as described above, the fixed rubber member would wear rapidly.
The Merila U.S. Pat. No. 4,394,021 describes a contact sealing which contacts a rotating sealing surface. The contact sealing includes a one-piece U-shaped housing creating an annular groove. A pressurized hose is located within the annular groove and abuts a sealing member to press the sealing member against the sealing surface.
A contact sealing as described in the Merila patent utilizes a one-piece housing with the pressurized hose mounted therein. Over a period of time, however, the hose and the sealing member will wear from use. The nature of the product matter being treated may also affect the life of the hose and/or the sealing member. In any event, when one or both of the hose and/or the sealing member is damaged, or otherwise impaired by wear or use, replacement is required. With the contact sealing as described in the Merila patent, the very nature of the hose being positioned within a one-piece housing renders the hose and the sealing member very difficult to remove from the housing and the rotary drum mixer. In fact, in order to effectuate repairs, the entire housing must be removed from the drum. Such a feat is tremendously labor intensive and rotary drum mixer "downtime" is dramatically increased. Additionally, during the replacement procedure, damage to the rotary drum mixer is a major concern.
Furthermore, the contact sealing as described in the Merila patent is intended for use on rotary drums in iron ore pelletizing plants, i.e., rotary drums having a diameter of between 12" to 36" operating in low purity applications. Due to the vast exposure of almost the entire contact sealing to the product matter being mixed thereby creating traps for product matter to collect, a dangerous, unhealthful environment could be present if the contact sealing of the Merila patent is used in the mixing or production of pharmaceuticals or food destined for human consumption, which is generally mixed in rotary drums having larger diameters.
A second potential problem with horizontally mounted drum mixers is the possibility of particle grinding between the rotating drum/scoop section combination and the stationary hood. When the drum mixer is used to mix hard abrasive particles such as in the mineral, glass and plastics industries, particle grinding can have very damaging effects. Particle grinding can significantly increase the power required to rotate the drum mixer, leading to increased load on the entire drum mixer drive system. Particle grinding can also lead to wear between the rotating drum/scoop section combination and the stationary hood, and can especially lead to wear of the seal between the rotating drum and the stationary hood. Wear can exacerbate the particle grinding and lead to failure of the seal. Additionally, particle grinding will reduce particle size of the product matter being mixed, which is often an undesired side effect of mixing.
Another potential problem with horizontally mounted drum mixers involves the cleanliness and complete emptying of the drum mixer. The discharge chute for drum mixers is often raised above the lowest point in the drum mixer. Raising the discharge chute allows for gravitational discharge into a container which is placed below the discharge chute. Complete emptying of the drum mixer, which is necessary both to avoid product waste and for cleaning of the drum mixer, is only accomplished if all the product matter in the mixer is adequately raised to the discharge chute.